In a Universal Mobile Telecommunications System (UMTS), such as that proposed for the next of the third generation partnership project (3GPP) standards for the UMTS Terrestrial Radio Access Network (UTRAN), such as wideband code division multiple access (WCDMA) or cdma2000 for example, user equipment (UE) such as a mobile station, (MS) communicates with any one or more of a plurality of base station subsystems (BSSs) dispersed in a geographic region. Typically, a BSS (known as Node-B in WCDMA) services a coverage area that is divided up into multiple sectors (known as cells in WCDMA). In turn, each sector is serviced by one or more of multiple base transceiver stations (BTSs) included in the BSS. The mobile station is typically a cellular communication device. Each BTS continuously transmits a downlink (pilot) signal. The MS monitors the pilots and measures the received energy of the pilot symbols.
In a cellular system, there are a number of states and channels for communications between the MS and the BSS. For example, in IS95, in the Mobile Station Control on the Traffic State, the BSS communicates with the MS over a Forward Traffic Channel in a forward link and the MS communicates with the BSS over a Reverse Traffic Channel in a reverse link. During a call, the MS must constantly monitor and maintain four sets of pilots. The four sets of pilots are collectively referred to as the Pilot Set and include an Active Set, a Candidate Set, a Neighbor Set, and a Remaining Set, where, although the terminology may differ, the same concepts generally apply to the WCDMA system.
The Active Set includes pilots associated with the Forward Traffic Channel assigned to the MS. This set is active in that the pilots and companion data symbols associated with this set are all actively combined and demodulated by the MS. The Candidate Set includes pilots that are not currently in the Active Set but have been received by the MS with sufficient strength to indicate that an associated Forward Traffic Channel could be successfully demodulated. The Neighbor Set includes pilots that are not currently in the Active Set or Candidate Set but are likely candidates for handoff. The Remaining Set includes all possible pilots in the current system on the current frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set.
When the MS is serviced by a first BTS, the MS constantly searches pilot channels of neighboring BTSs for a pilot that is sufficiently stronger than a threshold value. The MS signals this event to the first, serving BTS using a Pilot Strength Measurement Message. As the MS moves from a first sector serviced by a first BTS to a second sector serviced by a second BTS, the communication system promotes certain pilots from the Candidate Set to the Active Set and from the Neighbor Set to the Candidate Set. The serving BTS notifies the MS of the promotions via a Handoff Direction Message. Afterwards, for the MS to commence communication with a new BTS that has been added to the Active Set before terminating communications with an old BTS, a “soft handoff” will occur.
For the reverse link, typically each BTS in the Active Set independently demodulates and decodes each frame or packet received from the MS. It is then up to a switching center or selection distribution unit (SDU) normally located in a Base Station Site Controller (BSC), which is also known as a Radio Network Controller (RNC) using WCDMA terminology, to arbitrate between the each BTS's decoded frames. Such soft handoff operation has multiple advantages. Qualitatively, this feature improves and renders more reliable handoff between BTSs as a user moves from one sector to the adjacent one. Quantitatively soft-handoff improves the capacity/coverage in a WCDMA system. However, with the increasing amount of demand for data transfer (bandwidth), problems can arise.
Several third generation standards have emerged, which attempt to accommodate the anticipated demands for increasing data rates. At least some of these standards support synchronous communications between the system elements, while at least some of the other standards support asynchronous communications. At least one example of a standard that supports synchronous communications includes cdma2000. At least one example of a standard that supports asynchronous communications includes WCDMA.
While systems supporting synchronous communications can sometimes allow for reduced search times for handover searching and improved availability and reduced time for position location calculations, systems supporting synchronous communications generally require that the base stations be time synchronized. One such common method employed for synchronizing base stations includes the use of global positioning system (GPS) receivers, which are co-located with the base stations that rely upon line of sight transmissions between the base station and one or more satellites located in orbit around the earth. However, because line of sight transmissions are not always possible for base stations that might be located within buildings or tunnels, or base stations that may be located under the ground, sometimes the time synchronization of the base stations is not always readily accommodated.
However, asynchronous transmissions are not without their own set of concerns. For example, the timing of uplink transmissions in an environment supporting MS autonomous scheduling (whereby a MS may transmit whenever the MS has data in its transmit buffer and all MSs are-allowed to transmit as needed) by the individual MSs can be quite sporadic and/or random in nature. While traffic volume is low, the autonomous scheduling of uplink transmissions is less of a concern, because the likelihood of a collision (i.e. overlap) of data being simultaneously transmitted by multiple MSs is also low. Furthermore, in the event of a collision, there is spare bandwidth available to accommodate the need for any retransmissions. However, as traffic volume increases, the likelihood of data collisions (overlap) also increases. The need for any retransmissions also correspondingly increases, and the availability of spare bandwidth to support the increased amount of retransmissions correspondingly diminishes. Consequently, the introduction of explicit scheduling (whereby a MS is directed by the network when to transmit) by a scheduling controller can be beneficial.
However even with explicit scheduling, given the disparity of start and stop times of asynchronous communications and more particularly the disparity in start and stop times relative to the start and stop times of different uplink transmission segments for each of the non-synchronized base stations, gaps and overlaps can still occur. Both gaps and overlaps represent inefficiencies in the management of radio resources (such as rise over thermal (RoT), a classic and well-known measure of reverse link traffic loading in CDMA systems), which if managed more precisely can lead to more efficient usage of the available radio resources and a reduction in the rise over thermal (RoT).
For example, FIG. 1 is a block diagram of communication system 100 of the prior art. Communication system 100 can be a cdma2000 or a WCDMA system. Communication system 100 includes multiple cells (seven shown), wherein each cell is divided into three sectors (a, b, and c). A BSS 101–107 located in each cell provides communications service to each mobile station located in that cell. Each BSS 101–107 includes multiple BTSs, which BTSs wirelessly interface with the mobile stations located in the sectors of the cell serviced by the BSS. Communication system 100 further includes a radio network controller (RNC) 110 coupled to each BSS and a gateway 112 coupled to the RNC. Gateway 112 provides an interface for communication system 100 with an external network such as a Public Switched Telephone Network (PSTN) or the Internet.
The quality of a communication link between an MS, such as MS 114, and the BSS servicing the MS, such as BSS 101, typically varies over time and movement by the MS. As a result, as the communication link between MS 114 and BSS 101 degrades, communication system 100 provides a soft handoff (SHO) procedure by which MS 114 can be handed off from a first communication link whose quality has degraded to another, higher quality communication link. For example, as depicted in FIG. 1, MS 114, which is serviced by a BTS servicing sector b of cell 1, is in a 3-way soft handoff with sector c of cell 3 and sector a of cell 4. The BTSs associated with the sectors concurrently servicing the MS, that is, the BTSs associated with sectors 1-b, 3-c, and 4-a, are known in the art as the Active Set of the MS.
Referring now to FIG. 2, a soft handoff procedure performed by communication system 100 is illustrated. FIG. 2 is a block diagram of a hierarchical structure of communication system 100. As depicted in FIG. 2, RNC 110 includes an ARQ function 210, a scheduler 212, and a soft handoff (SHO) function 214. FIG. 2 further depicts multiple BTSs 201–207, wherein each BTS provides a wireless interface between a corresponding BSS 101–107 and the MSs located in a sector serviced by the BSS.
When performing a soft handoff, each BTS 201, 203, 204 in the Active Set of the MS 114 receives a transmission from MS 114 over a reverse link of a respective communication channel 221, 223, 224. The Active Set BTSs 201, 203, and 204 are determined by SHO function 214. Upon receiving the transmission from MS 114, each Active Set BTS 201, 203, 204 demodulates and decodes the contents of a received radio frame.
At this point, each Active Set BTS 201, 203, 204 then conveys the demodulated and decoded radio frame to RNC 110, along with related frame quality information. RNC 110 receives the demodulated and decoded radio frames along with related frame quality information from each BTS 201, 203, 204 in the Active Set and selects a best frame based on frame quality information. Scheduler 212 and ARQ function 210 of RNC 110 then generate control channel information that is distributed as identical pre-formatted radio frames to each BTS 201, 203, 204 in the Active Set. The Active Set BTSs 201, 203, 204 then simulcast the pre-formatted radio frames over the forward link.
Alternatively, the BTS of the current cell where the MS is camped (BTS 202) can include its own scheduler and bypass the RNC 110 when providing scheduling information to the MS. In this way, scheduling functions are distributed by allowing a mobile station (MS) to signal control information corresponding to an enhanced reverse link transmission to Active Set base transceiver stations (BTSs) and by allowing the BTSs to perform control functions that were previously supported by a RNC. The MS in a SHO region can choose a scheduling assignment corresponding to a best Transport Format and Resource Indicator information (TFRI) out of multiple scheduling assignments that the MS receives from multiple Active Set BTS. As a result, the enhanced uplink channel can be scheduled during SHO, without any explicit communication between the BTSs. In either case, explicit transmit power constraints (which are implicit data rate constraints) are provided by a scheduler, which are used by the MS 114, along with control channel information, to determine what transmission rate to use. MS buffer occupancy is also a parameter that is considered in determining a transmission rate.
As proposed for the UMTS system, a MS can use an enhanced uplink dedicated transport channel (EUDCH) to achieve an increased uplink data rate. The MS must determine the data rate to use for the enhanced uplink based on local measurements at the MS, such as buffer occupancy for example, and information provided by the scheduler.
In practice, when an MS is explicitly scheduled (Explicit Mode) by the BTS, for example, to use the enhanced uplink channel, or when a MS autonomously decides when to transmit data (Autonomous mode), the MS must determine a transmission rate given the constraints of a maximum rate or equivalently a maximum power margin indicated by the scheduler and the amount of data in its buffer. This is particularly important when the MS is in a multi-coverage area served by multiple cells where, in a CDMA system, such a MS is typically in soft handoff (SHO) with any of the said multiple cells if more than one are members of the MS's current Active Set.
The scheduling assignment is based on scheduling information that is sent by the mobile including buffer occupancy (BO), which is the amount of data in buffer that is to be transmitted in the uplink. In order to take advantage of diversity it is preferable to operate the enhanced uplink in SHO. When in SHO, the MS may get a scheduling assignment message from a BTS, and may successfully transmit data to this scheduling BTS. The other members of the active set may or may not be aware of this transaction depending on the relative strength of the uplink legs. This however potentially leads to several issues. Firstly, a non-scheduling BTS may schedule the MS based on outdated buffer occupancy reports received prior to the last successful scheduling of the MS. In addition, due to multiple schedulers that do not communicate with each other, the fairness of the overall system in scheduling users on the uplink may not be achieved. Further, the system will be skewed towards MSs reporting high BO with the potential for multiple BTSs scheduling the MS, resulting in a very unfair overall uplink throughput and there may also possibly be many wasted scheduling opportunities due to scheduling collisions between members of this MS's active set.
One solution is to have the MS include the BO in the TFRI sent on the uplink when scheduled. Since the TFRI is CRC protected this is a reliable mechanism for updating the BTSs. The TFRI includes information required by the BTS to decode the data channel and is sent in a separate message on an UL control channel, with the data sent on the enhanced uplink dedicated channel shortly following the control channel. However, it may be required that the TFRI be sent at a high enough power to ensure successful receipt at all active set Node Bs. This is a brute force approach especially in the presence of uplink imbalance in which case the other BTSs may not be using the BO report in any case due to the bad radio conditions and therefore unfavorable scheduling environment. Further, the BTS needs to be aware of the success of the data transmission linked to this TFRI message for the update to be successful, i.e. it needs to know if the data sent was successfully received. This is a problem since it is possible that the BTS receives the TFRI successfully but not the data. In this case, the BTS cannot reliably determine whether the MS was successful in the transmission or not depending on whether other BTSs in the active set received the data correctly or not in case it did not.
Another solution is to have more frequent BO reports. The frequency of the BO reporting can be increased to ensure that all the BTSs in the active set receive the latest BO status. However, this is not useful since it increases the uplink control messaging and reduces battery life and increases processing at the BTS. Another solution is to restrict non-scheduling BTSs from scheduling the MS until it receives the BO report again. This technique is rather limiting and can negatively impact system throughput in that the BTSs cannot take advantage of good radio conditions to schedule the MA. Further, due to the rapidly changing radio conditions, updates will have to be sent frequently on the MS to enable the BTSs to respond to radio conditions and MS movements. In addition, a BTS would need to schedule a MS immediately after the report is received to be sure that the BO report is meaningful.
Another solution is to include the BO in the data channel as part of the header. This avoids the problem described in the first solution described above. However, in this case only the successfully receiving BTS would be aware of the latest buffer occupancy. It does however help in avoiding the TFRI channel from being burdened with information that is not needed for the successful decoding of the data linked to the TFRI message. In addition, the receiving BTS could in principle determine the buffer status through combination of the traffic volume successfully received and the last BO report. However, this solution does not account for the change in buffer occupancy due to new data being generated at the transmitter. Even in this case by definition, the non-receiving BTSs in the active set do not have the latest BO report from the MS. A variant of the information in the header could be the sending of the rate of increase of BO as opposed to the actual BO. Or another variant is mobile could send the BO and include one bit as a rate indicator. However, this does lower data throughput.
Therefore, a need exists for a new technique to ensure that BTSs receive reliable buffer occupancy information. In particular, it would be of benefit to set up a technique to provide timely BO information to the active set BTSs such that a macro selection diversity benefit is obtained.